(a) Field of the Invention
The present invention relates to a beam index type color cathode ray tube (CRT), and more particularly, to an improved beam index type color CRT capable of displaying images much more clearly by preventing a deterioration of the contrast of displayed images.
(b) Description of the Related Art
A shadow mask cathode ray tube (CRT) is generally employed as an image displaying unit for televisions, computer monitors, etc. because of its low cost and relative clear image quality.
However, the shadow mask CRT exhibits low beam current efficiencies because a large portion of the beam current is blocked from striking the phosphor and is absorbed by the shadow mask. Such beam current absorbed by the shadow mask forms high current areas on the shadow mask, which causes thermal expansion on the shadow mask, resulting in deterioration of color definition of the CRT.
Also, because the shadow mask CRT is susceptible to effects from the earth's magnetic field, an inner shield for intercepting the earth's magnetic field must be provided in the CRT.
To overcome these shortcomings of the shadow mask type CRT, beam index type color CRTs without a shadow mask and inner shield have been developed.
FIG. 1 shows a conventional beam index type color CRT. As shown in FIG. 1, the beam index type color CRT includes a panel assembly 10 consisting of a screen panel 20 (in which phosphor is deposited) and a funnel 30, an electron gun 11 installed inside the funnel for emitting electron beams, a deflection yoke 12 mounted around a neck of the funnel 30 for deflecting the electron beams from the electron gun 11, an index beam (an ultra-violet ray) condenser plate 13 mounted on the curved part of the funnel 30, and photo sensor 14 for detecting and multiplying the index beam.
FIG. 2 shows an example of a conventional beam index type color CRT. As shown in FIG. 2, the beam index type color CRT includes a photo sensor 14 which detects the index beam which is generated from the beam index type color CRT upon impingement of the electron beam on index stripes thereof and responsively generates an index signal (an electric signal) depending on an amount of the detected index beam, a band pass filter (BPF) 20 for filtering a predetermined frequency of the index signals, a limiter 21 for limiting a predetermined amplitude of the index signals filtered by the filter 20, a phase-locked loop (PLL) 22 for synchronizing an index signal to color selecting signals, a start-set controller 23 for setting an index starting signal in accordance with the index signal filtered by the band pass filter 20, a ring counter 24 for counting each cycle of the filtered index signals, a color signal switch 25 for generating a color signal according to a index signal counted by ring counter 24, a run-in area controller 26 for controlling a run-in area on the screen panel, an index processing circuit 27 for synchronizing the color signal with the index signal and producing a synchronized signal, and an amplifier 28 for amplifying the synchronized signal and sending the amplified signal to the electron gun.
As represented in FIGS. 3 and 4, in the conventional beam index type color CRT, the inner surface of the screen panel 31 has parallel triads of red, green, and blue color phosphor stripes 33 vertically arranged with a black layer 32 between the adjacent color phosphor stripes 33. Index stripes 35 are provided on a transparent aluminum layer 34 which functions as a beam reflecting layer deposited on the triads of red, green, and blue (RGB) color phosphor stripes 33 and black layer 32.
On the one hand, in order to obtain the index signal as well as color signal, the index stripes must be constantly exposed to the electron beam in the beam index type CRT.
That is, the electron beam scans the screen panel even when the generation of color signals are not required for a dark image construction, such as when displaying a night scene.
Since the electron beam must constantly scans the screen panel for generating an index signal, even when a color signal is not required for a portion of the display panel, the constantly scanning electron beam activates the red, green, and blue phosphors of the portion such that unintended light emissions of the phosphors lowers the contrast of a beam Index type color CRT.